Try giving it a reset. Hold down the sleep and home keys, past when you see the red power down slider and until you see the silver apple. Let it reboot and try again.
Also check the connections. Is the plug in all the way? Do you hear the little chirp when it's plugged in?
It's possible that you have a defective charger or cord. So if you have others, try swapping them out and see if htat fixes it.